I have to make a program for school, need help.

im sure i did this program on exam and worked but i dont remember how it was done. So if anyone would like to help .....

task 1: to print all real numbers from: a) n to 2n; b) 2n to n;
the user need to enter a number for var n.
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task 2: to calculate arithmetic mean of the first n natural numbers.
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task 3: to calculate: 1+4+7+11+...+n.

i hope you would help me and thanks in advance.